Transmission scheduling for multi-stroke engine

ABSTRACT

A multi-mode internal combustion engine is described. As one example, the engine may be transitioned between a two stroke mode and a four stroke mode in coordination with a transmission shift by adjusting a number of strokes performed by the engine per combustion cycle while shifting the transmission between different gear ratios.

BACKGROUND AND SUMMARY

Some internal combustion engines may be operated in a four stroke mode,whereby the engine is operated to perform a combustion cycle once everyfour piston strokes. Other internal combustion engines may be operatedin a two stroke mode, whereby the engine is operated to perform acombustion cycle once every two piston strokes. Engines may producegreater maximum torque when operated in the two stroke mode as comparedto the four stroke mode since the combustion frequency and therefore thepower density of the engine may be greater in the two stroke mode thanthe four stroke mode. However, engines may demonstrate greater fuelefficiency when operated in the four stroke mode as compared to the twostroke mode, due in part to better air and fuel mixing before combustionof the charge is initiated.

Improvements in valve actuation systems have enabled engines to beoperated in the two stroke mode under some conditions and the fourstroke mode in other conditions. This approach has enabled improvementsin both fuel efficiency and increased maximum torque of the engine.However, the inventors herein have identified some issues with the thisapproach. As one example, transmission shifting may be increased as aresult of the multi-mode engine capability. Increased transmissionshifting may result in reduce fuel efficiency, increased transmissionwear, and reduced drivability of the vehicle as the transmission shiftsmay be perceived by the vehicle operator.

The inventors have recognized that transmission shifting may be reducedin some examples by a method of operating a propulsion system for avehicle, where the propulsion system includes an internal combustionengine coupled to one or more drive wheels of the vehicle via atransmission. As a non-limiting example, the method includes: operatingthe engine to produce an engine output; transferring the engine outputto one or more drive wheels of the vehicle via the transmission; andadjusting a number of strokes performed by the engine per combustioncycle while shifting the transmission between different gear ratios. Forexample, the engine may be transitioned from the four stroke mode to thetwo stroke mode while the transmission is up-shifted by reducing thegear ratio of the transmission. In this way, the transmission may beshifted by a greater extent (e.g. to a higher gear ratio) than wouldotherwise be suitable when the engine is operated in the four strokemode, while the new gear ratio may be suitable if the engine istransitioned to the two stroke mode providing increased maximum torquepotential.

The inventors herein have further recognized that the engine lug limit(i.e. the lower speed limit of the engine where NVH becomesunacceptable) may differ depending on whether the engine is operating inthe four stroke mode or the two stroke mode. For example, the inventorshave recognized that the lug limit may be lower in the two stroke modethan the four stroke mode as a result of the increased combustionfrequency of two stroke operation. As such, a method of operating apropulsion system for a vehicle is described. When the engine isrotating at a lower engine rotational speed, the vehicle speed may bereduced by increasing a number of strokes performed by the engine percombustion cycle while increasing a gear ratio of the transmission; andwhen the engine is rotating at a higher engine rotational speed, thevehicle speed may be reduced by reducing a number of strokes performedby the engine per combustion cycle while maintaining the transmission ata selected gear ratio.

In this way, transmission shifting during deceleration of the vehiclemay be delayed or eliminated, under some conditions, by transitioningthe engine from the four stroke mode to the two stroke mode as theengine rotational speed approaches the lug limit of the engine operatingin the four stroke mode rather than performing a transmission shift. Theuse of coordinated mode transitions may enable the transmission mayremain in the selected gear ratio for a longer period of time duringdeceleration of the vehicle, thereby enabling a subsequent up-shift ofthe transmission to be eliminated when the vehicle operator requestsacceleration of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example propulsion system for a vehicle.

FIGS. 2-4 illustrate flow charts depicting example process flows thatmay be used to control the propulsion system of FIG. 1.

FIG. 5 depicts an example transmission shift schedule.

FIG. 6 illustrates a flow chart depicting an example process flow thatmay be used to control the propulsion system of FIG. 1.

FIGS. 7A and 7B show example timelines depicting operation of an examplecylinder of the propulsion system in a two stroke cycle and a fourstroke cycle, respectively.

FIG. 8 depicts a detailed view of an example engine cylinder of thepropulsion system of FIG. 1.

FIGS. 9A, 9B, and 9C are timelines depicting how the combustion mode ofthe engine may be adjusted during a transmission shift.

DETAILED DESCRIPTION

FIG. 1 illustrates a propulsions system for a vehicle including aninternal combustion engine 100. Engine 100 may be operatively coupledwith one or more vehicle drive wheels indicated schematically at 130 viaa transmission 140. Engine 100 may include one or more combustionchambers or cylinders 110, a non-limiting example of which is depictedschematically in FIG. 8. In the example propulsion system of FIG. 1,engine 100 includes a total of eight cylinders. Engine 100 may includeother suitable numbers of cylinders in other embodiments, including 2,3, 4, 5, 6, 10, or 12 cylinders. A control system 120 may be operativelyand communicatively coupled with engine 100 as well as transmission 140,drive wheel 130, and other suitable components of the vehicle propulsionsystem.

In some embodiments, transmission 140 may be configured as acontinuously variable transmission (CVT), whereby the gear ratio of thetransmission may be increased or decreased continuously across a gearratio range. In other embodiments, transmission 140 may include aplurality of discrete gear ratios that may be selected by the vehicleoperator or the control system. For example, the gear ratio of thetransmission may be adjusted by shifting the transmission between two ormore of the discrete gear ratios provided by the transmission.

Engine cylinders 110 may be selectively operated one of a plurality ofdifferent combustion modes depending on operating conditions encounteredby the propulsion system. As a non-limiting example, engine cylinders110 may be operated in a two stroke cycle under some conditions, and maybe operated in a four stroke cycle under other conditions. A two strokemode corresponds to a combustion cycle where two piston strokes areutilized to carry out combustion, whereas a four stroke mode correspondsto a combustion cycle where four piston strokes are utilized to carryout combustion.

Since combustion occurs ever two piston strokes in the two stroke mode,the engine can produce substantially greater engine torque in the twostroke mode as compared to the four stroke mode. Specifically, therelative combustion frequency and power density of the engine may beincreased by transitioning the engine from the four stroke mode to thetwo stroke mode. Conversely, the relative combustion frequency and powerdensity of the engine may be reduced by transitioning the engine fromthe two stroke mode to the four stroke mode. An example of each of thesemodes is depicted in FIGS. 7A and 7B, respectively. In otherembodiments, the engine cylinders may be configured to operate in otheror different combustion modes, including six stroke modes or eightstroke modes.

FIG. 2 illustrates a flow chart depicting an example process flow thatmay be performed by control system 120. The process flow of FIG. 2depicts how transitions between the two stroke mode and the four strokemode may be coordinated with transmission shifting to enable thedelivery of a suitable wheel torque for propelling the vehicle.

At 210, one or more operating conditions may be assessed by the controlsystem. These operating conditions may include one or more of enginerotational speed, vehicle speed, an indication of requested wheel torque(e.g. as requested by a vehicle operator), transmission state (e.g.selected gear), current combustion mode of the engine, engine throttleposition, engine torque reserve for the current combustion mode, ambientconditions, an indication of engine noise vibration and harshness (NVH),among other operating conditions described with reference to FIG. 8.

As a non-limiting example, the control system may identify a request toincrease or decrease the wheel torque of the vehicle. This requestedincrease or decrease in wheel torque may be communicated to the controlsystem via a user input device such as an accelerator pedal depicted inFIG. 8 at 872.

At 212, it may be judged whether the control system should respond tothe requested change in wheel torque by adjusting both the selected gearof the transmission and the combustion mode of the engine. If the answerat 212 is judged yes, the process flow may proceed to 214, where theselected transmission gear and the combustion mode of the engine may beadjusted in response to the operating conditions identified at 210.

As a first example, the control system may perform a down-shift of thetransmission (i.e. increase the gear ratio) while transitioning theengine from a two stroke mode to a four stroke mode. By down-shiftingthe transmission (e.g. increasing the gear ratio) the speed of theengine may be increased through the increased gear ratio, whiletransitioning the engine to the four stroke mode can be used to reducethe amount of torque produced by the engine. This approach may be usedby the control system to respond to a requested increase or decrease inthe torque delivered to the drive wheels of the vehicle as willdescribed in greater detail with reference to FIGS. 3 and 4.

As a second example, the control system may down-shift the transmissionwhile transitioning the engine from the four stroke mode to the twostroke mode. The down-shift serves to increase the engine rotationalspeed, while the transition to the two stroke mode further increases thetorque reserve of the engine. This approach may be used by the controlsystem to respond to a requested increase in the torque delivered to thedrive wheels of the vehicle.

As a third example, the control system may perform an up-shift of thetransmission (i.e. reduce the gear ratio) while transitioning the enginefrom the two stroke mode to the four stroke mode. The up-shift of thetransmission serves to reduce the engine rotational speed while thetransition to the four stroke mode can reduce the torque reserve whileincreasing engine efficiency.

As a fourth example, the control system may perform an up-shift of thetransmission while transitioning the engine from the four stroke cycleto the two stroke cycle. Depending on operating conditions, thisapproach may be used by the control system to respond to a requestedincrease or decrease in the torque delivered to the drive wheels of thevehicle as will described in greater detail with reference to FIGS. 3and 4.

Alternatively, if the answer at 212 is judged no, the process flow mayinstead proceed to 216. At 216, it may be judged whether to respond to achange in the requested wheel torque by adjusting a combustion mode ofthe engine without adjusting the selected gear of the transmission. Ifthe answer at 216 is judged yes, the process flow may proceed to 218,where the combustion mode of the engine may be adjusted in response tothe requested change in wheel torque, while maintaining the currentlyselected transmission gear.

As a first example, the control system may transition the engine fromthe two stroke mode to the four stroke mode, while maintaining thecurrently selected gear ratio. This approach may be used by the controlsystem to respond to a requested decrease in the wheel torque (e.g.tip-out). As a second example, the control system may transition theengine from the four stroke mode to the two stroke mode, whilemaintaining the currently selected gear ratio. This approach may be usedby the control system to respond to a requested increase in the wheeltorque (e.g. tip-in).

Alternatively, if the answer at 216 is judged no, the process flow mayproceed to 220. At 220, it may be judged whether to respond to a changein the requested wheel torque by adjusting the selected gear of thetransmission without adjusting the combustion mode of the engine. If theanswer at 220 is judged yes, the process flow may proceed to 222, wherethe selected transmission gear may be adjusted while maintaining thecurrent combustion mode of the engine.

As a first example, the control system may down-shift the transmission,while maintaining the engine in the two stroke cycle. As a secondexample, the control system may down-shift the transmission, whilemaintaining the engine in the four stroke cycle. These approaches may beused by the control system to respond to a requested increase in thetorque delivered to the drive wheels of the vehicle. As a third example,the control system may up-shift the transmission, while maintaining theengine in the two stroke cycle. As a fourth example, the control systemmay up-shift the transmission, while maintaining the engine in the fourstroke cycle.

From 214, 218, 222, and 224, the process flow may proceed to 224. At224, the air and fuel delivered to the engine cylinders may be adjustedto respond to the change in requested wheel torque. From 214, 218, and222, the adjustment of the air and fuel performed at 224 may be inaddition to the adjustments made to the transmission gear and/or enginecombustion mode. From 220, the adjustment of the air and fuel performedat 224 may be performed while maintaining the engine combustion mode andselected transmission gear.

As a first example, the air and fuel delivered to the engine cylindersmay be increased at 224 to respond to a requested increase in wheeltorque, thereby increasing the amount of torque produced by the engine.As another example, the air and fuel delivered to the engine cylindersmay be decreased to respond to a requested decrease in wheel torque,thereby decreasing the amount of torque produced by the engine.

The adjustment to the air and fuel delivered to the engine may beperformed in accordance with a prescribed air/fuel ratio or air/fuelratio range. For example, the control system may increase or decreasethe amount of air and fuel delivered to the engine cylinders whilemaintaining a stoichiometric air/fuel ratio, or other suitable ratio.From 224, the process flow may return to 210.

In this way, FIG. 2 provides at least some of the various operationsthat may be performed by the propulsion system, whereby transmissionstate and combustion mode of the engine may be coordinated to providethe requested wheel torque to the drive wheels at a given vehicle speed.As will be described in the context of FIG. 5, the engine may beoperated in the two stroke mode during a first operating condition andmay be operated in the four stroke mode during a second operatingcondition. As a non-limiting example, the engine may be operated in thetwo stroke mode at lower engine rotational speeds and may be operated inthe four stroke mode at higher engine rotational speeds.

FIG. 3 illustrates a flow chart depicting an example process flow thatmay be performed in response to a vehicle operator tip-in or requestedvehicle acceleration. The process flow of FIG. 3 provides a non-limitingexample of the more general process flow depicted in FIG. 2.

At 310 it may be judged whether a vehicle operator tip-in has occurredor whether vehicle acceleration is requested. For example, the controlsystem may judge whether a tip-in has occurred based on input from apedal position sensor as depicted in FIG. 8. If the answer at 310 isjudged no, the process flow may return. Alternatively, if the answer at310 is judged yes, the process flow may proceed to 312. At 312, it maybe judged whether the engine is currently operating in the two strokemode. If the answer at 312 is judged yes, the process flow may proceedto 314.

At 314, it may be judged whether to transition the engine from the twostroke mode to the four stroke mode. The decision to transition theengine to the four stroke mode at 314 may be based on one or more of themagnitude of the tip-in or requested acceleration identified at 310 andat 210, the torque production limits of the engine operating in fourstroke mode, the lug limits of the engine while operating in the fourstroke mode and two stroke mode, and the over speed limits of the enginewhile operating in the four stroke mode and the two stroke mode.

As one example, the answer at 314 may be judged no in response to alarger tip-in or a larger requested increase in vehicle speed, and theanswer at 314 may be judged yes in response to a smaller tip-in or asmaller requested increase in vehicle speed. Maintaining the engine inthe two stroke cycle can enable the engine to output more torque andtransitioning the engine to the four stroke cycle can enable the engineto output less torque, while the wheel torque may be increased bydown-shifting the transmission.

If the answer at 314 is judged no, the process flow may proceed to 316,where the propulsion system may responds to the tip-in or requestedacceleration by down-shifting the transmission while maintainingoperation of the engine in the two stroke mode. As the transmission isdown-shifted, the wheel torque may be increased while maintaining theengine in the current combustion mode.

Alternatively, if the answer at 314 is judged yes (i.e. a transition tothe four stroke cycle is to be performed) the process flow may proceedto 318. At 318, the engine may be transitioned to the four stroke modeand the transmission may be down-shifted in response to the tip-in orvehicle acceleration request. Returning to 312, if the answer isalternatively judged no (i.e. the engine is not currently operating inthe two stroke mode) the process flow may proceed to 320. At 320, it maybe judged whether the engine is currently operating in the four strokemode. If the answer at 320 is judged yes, the process flow may proceedto 322.

At 322 it may be judged whether to transition the engine from the fourstroke cycle to the two stroke cycle. The decision to transition theengine to the two stroke mode at 322 may be based on one or more of themagnitude of the tip-in or requested acceleration identified at 310 andat 210, the torque production limits of the engine operating in fourstroke mode, the lug limits of the engine while operating in the fourstroke mode and two stroke mode, and the over speed limits of the enginewhile operating in the four stroke mode and the two stroke mode.

As one example, the control system may judge whether the tip-in orrequested acceleration may be achieved by adjusting only the combustionmode, by adjusting only the transmission gear, or by adjusting both thecombustion mode and the transmission gear. The answer at 322 may bejudged in response to the magnitude of the tip-in or requestedacceleration identified at 310 or 210.

For example, if a transition to the two stroke mode is not to beperformed, the process flow may proceed to 324. At 324, the four strokemode may be maintained by the engine and the transmission may bedownshifted. Alternatively, if the answer at 322 is yes, the processflow may proceed to 326 where it may be judged whether to down-shift thetransmission. As one example, the control system may identify whether adown-shift is to be performed in response to one or more of themagnitude of the tip-in, the torque production limits of the engineoperating in the two stroke mode, the lug limit of the engine operatingin the two stroke mode, and the over speed limit of the engine operatingin the two stroke mode.

For example, if the engine cannot produce sufficient torque whileoperating in the two stroke mode to meet the requested wheel torque asindicated by the tip-in, the transmission may be down-shifted. Asanother example, if the down-shift would increase the engine rotationalspeed beyond the over speed limit of the engine while operating in thetwo stroke mode, the down-shift may not be performed.

If the answer at 326 is judged yes, process flow may proceed to 328. At328, the engine may be transitioned to the two stroke mode from the fourstroke mode, and the transmission may be down-shifted. The operation at328 may be performed by the control system in response to a greatertip-in or larger requested increase in vehicle speed as compared to theoperation at 324, since the two stroke mode can provide greater engineoutput torque than the four stroke mode. Alternatively, if the answer at326 is judged no (i.e. a transmission down-shift is not to beperformed), the process flow may proceed to 330.

At 330, it may be judged whether to perform an up-shift of thetransmission in conjunction with the transition to the two stroke mode.The control system may judge whether to perform an up-shift at 330 inresponse to one or more of the magnitude of the tip-in, the torqueproduction limits of the engine operating in the two stroke mode, thelug limit of the engine while operating in the two stroke mode, and theover speed limit of the engine while operating in the two stroke mode.

For example, if the engine rotational speed is greater than the overspeed limit for the two stroke mode before the engine is transitioned tothe two stroke mode, the control system may judge that an up-shift is toaccompany with the transition. As another example, if the enginerotational speed after the up-shift would exceed the lug limit of theengine while operating in the two stroke mode, the up-shift may not beperformed.

If the answer at 330 is judged yes, the process flow may proceed to 332.At 332, the transmission may be up-shifted and the engine may betransitioned to the two stroke mode from the four stroke mode. Thetransition from the four stroke mode to the two stroke mode enables thepropulsion system to achieve the requested wheel torque indicated by thetip-in, even when the transmission is up-shifted.

Alternatively, if the answer at 330 is judged no (i.e. an up-shift isnot to be performed), the process flow may proceed to 334. At 334, theengine may be transitioned to the two stroke mode from the four strokemode while maintaining the currently selected transmission gear, and theengine may be operated to achieve the requested wheel torque indicatedby the tip-in. From 316, 318, 324, 328, and 334, the process flow mayreturn to 310.

As described with reference to FIG. 3, the engine may be operated tocarry out combustion in a four stroke mode, where in response to adriver tip-in, the engine may be transitioned to a two stroke mode fromthe four stroke mode and the gear ratio of the transmission may beadjusted. The gear ratio may be adjusted in some examples by up-shiftingthe transmission as shown at 332 or may be down-shifted in some examplesas shown at 328. After these transitions are performed, the engine maybe operated to carry out combustion in the two stroke mode to increasethe level of torque delivered to drive wheels via the transmission inaccordance with the driver tip-in.

FIG. 4 illustrates a flow chart depicting an example process flow thatmay be performed in response to a vehicle operator tip-out or requestedvehicle deceleration. The process flow of FIG. 4 provides anothernon-limiting example of the more general process flow depicted in FIG.2.

At 410 it may be judged whether a vehicle operator tip-out has occurredor whether vehicle deceleration is requested. For example, the controlsystem may judge whether a tip-out has occurred based on input from apedal position sensor as depicted in FIG. 8. If the answer at 410 isjudged no, the process flow may return. Alternatively, if the answer at410 is judged yes, the process flow may proceed to 412. At 412, it maybe judged whether the engine is currently operating in the two strokemode. If the answer at 412 is judged no, the process flow may proceed to414.

At 414, the four stroke mode may be maintained while the vehicle isdecelerated or tip-out has occurred. At 416, it may be judged whetherthe engine lug limit of the four stroke mode has been reached. Forexample, the control system may assess the engine rotational speed aspreviously described with reference to 210, whereby engine rotationalspeed may be compared to a lug limit for the four stroke mode that maybe stored in memory at the control system. As a non-limiting example,the lug limit for the four stroke mode may be approximately 1100 rpm, orother suitable engine rotational speed.

If the answer at 416 is judged no (e.g. the engine rotational speed isapproaching or is greater than the lug limit for the four stroke mode),the process flow may return to 414 where the four stroke mode ismaintained throughout the tip-out or vehicle deceleration.Alternatively, if the answer at 416 is judged yes, the process flow mayproceed to 418. At 418, the engine may be transitioned to the two strokemode while the current transmission gear is maintained. Since thecombustion frequency of the engine operating in the two stroke mode isapproximately twice that of the engine operating in the four strokemode, the lug limit for the two stroke mode may be approximately half ofthe lug limit of the four stroke mode. For example, where the lug limitin the four stroke mode is 1100 rpm, the lug limit in the two strokemode may be 550 rpm. By transitioning the engine from the four strokemode to the two stroke mode, the engine may prolong operation in thecurrently selected gear during a deceleration of the vehicle.

In this way, when the engine is rotating or operating at a lower enginerotational speed, the vehicle speed may be reduced by increasing anumber of strokes performed by the engine per combustion cycle (e.g.transition from two stroke to four stroke) while increasing a gear ratioof the transmission (e.g. down-shifting); and when the engine isrotating or operating at a higher engine rotational speed, the vehiclespeed may be reduced by reducing a number of strokes performed by theengine per combustion cycle (e.g. transition from four stroke to twostroke) while maintaining the transmission at a selected gear ratio.

As will be described with reference to FIG. 5, a similar approach may beused for increasing vehicle speed. For example, when the engine isrotating at a higher engine rotational speed, the vehicle speed may beincreased by reducing the number of strokes performed by the engine percombustion cycle while reducing a gear ratio of the transmission (e.g.up-shifting); and when the engine is rotating at the lower enginerotational speed, the vehicle speed may be increased by increasing thenumber of strokes performed by the engine per combustion cycle whilemaintaining the transmission at the selected gear ratio.

From 418, the process flow may proceed to 420. At 420, it may be judgedwhether the lug limit has been reached while the engine is operating inthe two stroke mode. As described with reference to 416, the controlsystem may compare the current engine rotational speed to the lug limitfor the two stroke mode stored in memory to determine whether the enginerotational speed is approaching or exceeding the lug limit for theparticular combustion mode. If the answer at 420 is no, the process flowmay proceed to 430 where the two stroke mode may be maintained. From430, the process flow may return to 420.

Alternatively, if the answer at 420 is judged yes (e.g. the enginerotational speed is approaching or is less than the lug limit of the twostroke mode), the process flow may proceed to 422. At 422 and 424, thetransmission may be down-shifted and the engine may be transitioned tothe four stroke mode. In some embodiments, the control system may have apreference for the four stroke mode over the two stroke mode duringdeceleration of the vehicle in order to conserve fuel and further reduceengine torque. From 424, the process flow may return to 414.

Returning to 412, if the answer is alternatively judged no (i.e. thecurrent mode is the two stroke mode), the process flow may proceed to426. At 426, it may be judged whether the engine rotational speed isless than the lug limit of the four stroke mode. If the answer is judgedno, the engine may be transitioned to the four stroke mode at 428 toconserve fuel and reduce engine torque in response to the tip-out orduring the vehicle deceleration event. From 428, the process flow mayproceed to 414.

Alternatively, if the answer at 426 is judged yes (i.e. that enginerotational speed is approaching or below the lug limit of the fourstroke mode), the process flow may proceed to 430 where the two strokemode may be maintained. In this way, the control system may beconfigured to transition the engine to the four stroke mode in responseto a tip-out or requested vehicle deceleration if the lug limit of thefour stroke mode is not exceeded, while the two stroke mode may be usedto maintain the currently selected gear where downshift would otherwisebe performed. By maintaining the currently selected gear to a greaterextent, tip-outs of relatively short temporal nature may be responded towithout requiring a transmission shift. Thus, where a brief tip-out isfollowed by a subsequent tip-in, at least two transmission shifts may beavoided, namely a first down-shift from the initially selected gearfollowed by a second up-shift back to the initially selected gear may beeliminated by utilizing the two stroke mode.

FIG. 5 depicts example transmission shift schedules, which illustratethe process flow of FIGS. 2-4. A first shift schedule for the engineselectively operated in the two stroke mode and the four stroke mode isdepicted at 510 as a solid line. The second shift schedule for theengine operating in only the four stroke mode is depicted at 520 as abroken line.

At lower vehicle speeds, the engine may be operated in the four strokemode (4S) while the first gear transmission gear (1G) is selected. Athigher vehicle speeds, the transmission may be up-shifted (e.g. gearratio may be reduced). As illustrated by line 520, the transmission maybe up-shifted to a second gear (2G) while the engine continues tooperate in the four stroke cycle. By contrast, line 510 illustrates howa transition to the two stroke mode can enable the transmission to beup-shifted to a greater extent while delivering the same or similarlevel of wheel torque at a given engine rotational speed.

For example, the transmission may be up-shifted to third gear (3G)rather than second gear (2G) since the engine has sufficient torquereserve when operating in two stroke mode to enable the second gear (2G)to be optionally skipped. Therefore, by reducing the number of strokescarried out by the engine per combustion cycle in coordination with anup-shift, an intermediate gear of the transmission may be optionallyeliminated or omitted, thereby reducing the cost, weight, and complexityof the transmission as well as reducing the number of shifts that areperformed by the transmission during an acceleration event.

For example, if the engine is instead operated with a constant number ofstrokes per combustion event, such as with reference to the four strokecycle depicted by line 520, intermediate transmission gears may berequired to provide the requested wheel torque across a range of vehiclespeeds. Line 520 further depicts how the transmission may be up-shiftedto third gear (3G) at even higher speeds while the four stroke operationis maintained. By contrast, operating point 530 depicts how the enginemay be transitioned once again from the two stroke mode to the fourstroke mode for operation at higher engine rotational speeds.

Line 550 illustrates how engine rotational speed may vary in proportionto vehicle speed across a selected gear ratio such as 3G. As anon-limiting example, operating point 530 may represent the lug limit ofthe engine operating in the four stroke cycle, or may represent theshift point that is at an engine rotational speed that is greater thanthe lug limit of the engine while operating in the four stroke cycle. Asdepicted by line 550, the engine may be operated in the two stroke modeat lower engine rotational speeds and may be operated in the four strokemode at higher engine rotational speeds.

At even higher engine rotational speeds, transition of the engine fromthe four stroke mode (4S) to the two stroke mode (2S) depicted by line510 can enable the transmission to proceed directly from third gear (3G)to fifth gear (5G), thereby skipping fourth gear. Upon reduction of thegear ratio from 3G to 5G, the engine speed may be reduced and two strokemode may be performed. Since the engine is able to produce greaterengine torque in the two stroke mode than the four stroke mode, theengine can still deliver the requested wheel torque even when greaterreduction in the gear ratio are performed as a consequence of theup-shift.

By contrast, line 520 depicts how the four stroke engine utilizes anup-shift to an intermediate fourth gear (4G) before up-shifting again tothe fifth gear (5G) in order to deliver the requested wheel torqueacross the vehicle speed range. As shown at operating point 540 of line510, the multi-stroke engine may be transitioned once again from the twostroke mode (2S) to the four stroke mode (4S) while maintaining thetransmission state in fifth gear.

As can be demonstrated by FIG. 5, the vehicle speed may be increased byshifting the transmission differently depending on whether the engine istransitioned between the two stroke and four stroke modes or whether theengine maintains the four stroke mode. The example of FIG. 5, themulti-stroke operation provided by the transitions between four strokeand two stroke cycles enables a reduction in the number of shiftsperformed by the transmission (e.g. by skipping intermediate gears), andmay optionally enable at least some of the intermediate gears to beeliminated or omitted from the transmission.

In some embodiments, these intermediate gears may be retained in thetransmission so that failure or degradation of the engine's ability tobe transitioned between the four stroke mode and the two stroke mode canenable the engine to still deliver the requested wheel torque across theentire range of vehicle speeds by utilizes the intermediate gears asdescribed with reference to line 520.

The vehicle speed may be reduced in a similar manner as described abovewith reference to the vehicle speed increase. As one example, where thevehicle is initially operating at higher speeds, the engine may betransitioned to the two stroke mode from the four stroke mode as theengine approaches its lug limit in the four stroke mode, rather thanperforming a down-shift. For example, where the engine is initiallyoperating in the four stroke cycle (4S) and the transmission is in fifthgear (5G), the propulsion system can respond to a reduction in vehiclespeed by transitioning the engine to the two stroke cycle as indicatedby line 510 at 540. By contrast, line 520 depicts how the engineoperating exclusively in the four stroke mode may utilize a down-shiftof the transmission as indicated by the transition from fifth gear (5G)to fourth gear (4G).

Note that where the intermediate gears are retained in the transmission,the engine may be optionally operated in a four stroke mode duringdeceleration of the vehicle in order to increase fuel efficiency,whereby the intermediate gears are used to maintain the enginerotational speed within the lug limits and over speed limits of the fourstroke mode.

FIG. 5 further depicts how the transmission may be shifted between alower gear ratio and a higher gear ratio in response to a change of anoperating condition, such as vehicle speed, engine rotational speed, oroperator requested engine torque as indicated by accelerator pedalposition. Furthermore, FIG. 5 depicts how the number of strokesperformed by the engine per combustion cycle may be increased during ashift of the transmission from the lower gear ratio to the higher gearratio; and the number of strokes performed by the engine per combustioncycle may be reduced during a shift of the transmission from the lowergear ratio to the higher gear ratio.

FIG. 6 illustrates a flow chart depicting an example process flow thatmay be performed by the control system to transition the cylindersbetween a two stroke cycle and a four stroke cycle. At 612 it may bejudged whether to operate a cylinder of the engine in the two strokecycle in accordance with the process flow of FIGS. 2-4. If the answer at612 is yes, the control system may adjust one or more of the cylinder'sintake and/or exhaust valve timing at 614, the cylinder fueling at 616,and the ignition timing at 618 according to the two stroke cycle.

Alternatively, if the answer at 612 is judged no, the process flow mayproceed to 620 where it may be judged whether to operate the cylinder inthe four stroke cycle in accordance with the process flow of FIGS. 2-4.If the answer at 620 is yes, the process flow may adjust one or more ofthe cylinder's intake and/or exhaust valve timing at 622, the cylinderfueling at 616, and the ignition timing at 618 according to the fourstroke cycle. Furthermore, in some embodiments, throttle position may beadjusted in response to the particular operating mode of the engine tovary intake airflow supplied to the cylinders that are carrying outcombustion.

Referring also to FIGS. 7A and 7B, timing diagrams are depicted for anexample cylinder operating in a two stroke cycle and a four strokecycle, respectively. An indication of time is provided along thehorizontal axes of FIGS. 7A and 7B with reference to piston position.Top dead center (TDC) and bottom dead center (BDC) represent the pistonposition relative to the cylinder as it reciprocates throughoutoperation of the engine. A comparison of FIGS. 7A and 7B illustrates howthe intake and exhaust valves of the cylinder may be opened twice asoften or at a higher frequency in the two stroke cycle as the fourstroke cycle. Further, fuel may be delivered to the engine at twice thefrequency during the two stroke cycle as the during the four strokecycle. For example, the cylinder may be fueled approximately every 360crank angle degrees during the two stroke cycle and approximately every720 degrees during the four stroke cycle. Further still, ignition of theair and fuel charge within the cylinder may be performed around each TDC(e.g. approximately every 360 crank angle degrees) in the two strokecycle, and may be performed around every other TDC in the four strokecycle (e.g. approximately every 720 crank angle degrees).

FIG. 8 illustrates a schematic depiction of an example cylinder 110 ofengine 100 including the intake and exhaust system components thatinterface with the cylinder. Note that cylinder 110 may correspond toone or more of the previously described multi-stroke cylinders. Cylinder110 is at least partially defined by combustion chamber walls 810 andpiston 812. Piston 812 may be coupled to a crankshaft 816 via a crankarm, along with other pistons of the engine. Crankshaft 816 may beoperatively coupled with drive wheel 130 via transmission 140 asdepicted in FIG. 1.

Cylinder 110 may receive intake air via an intake passage 820. Intakepassage 820 may also communicate with other cylinders of engine 100.Intake passage 820 may include a throttle 842 including a throttle plate844 that may be adjusted by control system 120 to vary the flow ofintake air that is provided to the engine cylinders. Cylinder 110 cancommunicate with intake passage 820 via one or more intake valves 822.As a non-limiting example, these intake valves may be configured asintake poppet valves that are arranged near the top or upper region ofcylinder 110. However, in other embodiments, these intake valves may bearranged in a lower region of the cylinder.

An intake valve actuator 824 may comprise a valve actuation system thatis operatively coupled with one or more of the poppet intake valves.Intake valve actuator 824 may enable the opening and closing timing ofone or more of the intake valves to be adjusted to enable operation ofthe engine in the two stroke mode and the four stroke mode. For example,intake valve actuator 824 may be operated to increase the frequency atwhich the intake poppet valves are opened to perform the two stroke modeand reduce the frequency at which the intake poppet valves are opened toperform the four stroke mode. In some embodiments, intake valve actuator824 may include a cam profile switching device to enable different camsto be used to actuate the intake valves during each of the two strokemode and the four stroke mode. In other embodiments, intake valveactuator 824 may include an electromagnetic valve actuator (EVA) forcontrolling the opening and closing timing of one or more of the intakevalves.

Cylinder 110 may exhaust products of combustion via an exhaust passage830. Cylinder 110 can communicate with exhaust passage 830 via one ormore exhaust valves 832. As a non-limiting example, these exhaust valvesmay be configured as exhaust poppet valves that are arranged near thetop or upper region of cylinder 110. An exhaust valve actuator 834 mayfurther comprise the valve actuation system that is operatively coupledwith one or more of the poppet exhaust valves. Exhaust valve actuator834 may enable the opening and closing timing of one or more of theexhaust valves to be adjusted to enable operation of the engine in thetwo stroke mode and the four stroke mode. For example, exhaust valveactuator 834 may be operated to increase the frequency at which theexhaust poppet valves are opened to perform the two stroke mode andreduce the frequency at which the exhaust poppet valves are opened toperform the four stroke mode. In some embodiments, exhaust valveactuator 834 may include a cam profile switching device to enable theuse of different cams to actuate the exhaust valves during each of thetwo stroke mode and the four stroke mode. In other embodiments, exhaustvalve actuator 834 may include an electromagnetic valve actuator (EVA)for controlling the opening and closing timing of one or more of theexhaust valves.

In some embodiments, cylinder 110 may optionally include a spark plug856, which may be actuated by an ignition system 858. A fuel injector850 may be provided in the cylinder to deliver fuel directly thereto.However, in other embodiments, the fuel injector may be arranged withinintake passage 820 upstream of intake valve 822. Fuel injector 850 maybe actuated by a driver 852.

A non-limiting example of control system 120 is depicted schematicallyin FIG. 8. Control system 120 may include a processing subsystem (CPU)880, which may include one or more processors. CPU 880 may communicatewith memory, including one or more of read-only memory (ROM) 882,random-access memory (RAM) 884, and keep-alive memory (KAM) 886. Storagemedium read-only memory 882 can be programmed with computer readabledata representing instructions executable by processing subsystem 880for performing the methods and process flows described herein as well asother variants that are anticipated but not specifically listed.

CPU 880 can communicate with various sensors and actuators of engine 100via an input/output device 888. As a non-limiting example, these sensorsmay provide operating condition information to the control system, andmay include: an indication of mass airflow (MAF) through intake passage820 via sensor 846, an indication of manifold air pressure (MAP) viasensor 848, an indication of throttle position (TP) via throttle 842, anindication of engine coolant temperature (ECT) via sensor 854 which maycommunicate with coolant passage 814, an indication of engine rotationalspeed (PIP) via sensor 870, an indication of exhaust gas oxygen content(EGO) via sensor 838, an indication of a request or input (PP) fromvehicle operator 874 via position sensor 876 of pedal 872, an indicationof intake valve position via sensor 826, and an indication of exhaustvalve position via sensor 836, among others.

Furthermore, the control system may control operation of the engine 100,including cylinder 110 via one or more of the following actuators:driver 852 to vary fuel injection timing and quantity, ignition system858 to vary spark timing, intake valve actuator 824 to vary intake valvetiming, exhaust valve actuator 834 to vary exhaust valve timing, andthrottle 842 to vary the position of throttle plate 844, among others.Note that intake and exhaust valve actuators 824 and 834 may includeelectromagnetic valve actuators (EVA) and/or cam-follower basedactuators.

FIGS. 9A, 9B, and 9C are timelines depicting how the combustion mode ofthe engine may be adjusted during a transmission shift. In each of theseexamples, the transmission is adjusted from a first gear ratio to asecond gear ratio by an intermediate gear shift. The first gear ratiomay be lower or higher than the second gear ratio. During the gearshift, the engine combustion mode is transitioned from a mode where afirst number of strokes are performed per cycle to a mode where a secondnumber of strokes are performed per cycle via an intermediate modetransition. For example, the engine may be transitioned from a twostroke cycle to a four stroke cycle, or vice-versa. As shown in FIGS.9A, 9B, and 9C, the mode transition may be performed more rapidly thanthe gear shift, at least in some examples.

As shown in FIG. 9A, the mode transition may be initiated after the gearshift is initiated, where the mode shift is completed before the gearshift is completed. Alternatively, as shown in 9B, the mode shift may beinitiated before the gear shift is initiated, but before the gear shiftis completed, where the mode shift may be completed after the gear shiftis completed. Alternatively, as shown in FIG. 9C, the mode shift may beinitiated before the gear shift is initiated, where the mode shift maybe completed after the gear shift is initiated, but before the gearshift is completed. Thus, as demonstrated by FIGS. 9A, 9B, and 9C, themode transition may be performed during the gear shift.

Note that the example control and estimation process flows includedherein can be used with various engine and/or vehicle systemconfigurations. The process flows described herein may represent one ormore of any number of processing strategies such as event-driven,interrupt-driven, multi-tasking, multi-threading, and the like. As such,various acts, operations, or functions illustrated may be performed inthe sequence illustrated, in parallel, or in some cases omitted.Likewise, the order of processing is not necessarily required to achievethe features and advantages of the example embodiments described herein,but is provided for ease of illustration and description. One or more ofthe illustrated acts or functions may be repeatedly performed dependingon the particular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-8, V-10, V-12, opposed 4, and other engine types. Thesubject matter of the present disclosure includes all novel andnonobvious combinations and subcombinations of the various systems andconfigurations, and other features, functions, and/or propertiesdisclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

The invention claimed is:
 1. A method for a vehicle propulsion systemincluding an engine coupled to a drive wheel via a transmission,comprising: during lower engine speeds, reducing vehicle speed byincreasing a number of strokes performed by the engine per combustioncycle while increasing a transmission gear ratio; and during higherengine speeds, reducing vehicle speed by reducing a number of strokesperformed by the engine per combustion cycle while maintaining thetransmission gear ratio.
 2. The method of claim 1, where said reducingthe number of strokes includes transitioning the engine from a fourstroke cycle to a two stroke cycle.
 3. The method of claim 2, where saidincreasing the number of strokes includes transitioning the engine fromthe two stroke cycle to the four stroke cycle.
 4. The method of claim 1,further comprising: during the higher engine speeds, increasing thevehicle speed by reducing the number of strokes performed by the engineper combustion cycle while reducing the transmission gear ratio; andduring the lower engine speeds, increasing the vehicle speed byincreasing the number of strokes performed by the engine per combustioncycle while maintaining the transmission gear ratio.
 5. The method ofclaim 4, where increasing the number of strokes performed by the engineper combustion cycle is initiated after increasing the transmission gearratio is initiated and before increasing the transmission gear ratio iscompleted.
 6. A method for a vehicle system including an engine coupledto a transmission, comprising: downshifting the transmission based on anoperating condition; increasing a number of strokes performed by theengine per combustion cycle during a first transmission down-shift; andreducing a number of strokes performed by the engine per combustioncycle during a second transmission down-shift.
 7. The method of claim 6,where said increasing the number of strokes performed by the engine percombustion cycle includes transitioning the engine from a two strokecycle to a four stroke cycle, and wherein reducing the number of strokesperformed by the engine per combustion cycle includes transitioning theengine from the four stroke cycle to the two stroke cycle.
 8. The methodof claim 6, where the operating condition includes vehicle speed.
 9. Themethod of claim 6, where the operating condition includes enginerotational speed.
 10. The method of claim 6, where the operatingcondition includes an amount of wheel torque requested by a vehicleoperator as indicated by a position of an accelerator pedal.
 11. Themethod of claim 6, wherein downshifting the transmission includesshifting the transmission from a lower gear ratio to a higher gearratio.